A known thermal fuse for electrical equipment with a base part made of insulating material is described in German Patent 28 26 205. On one side of the base part, there are a fixed contact and a moving contact fastened to the free end of a contact spring, and on the other side of the base part, there is a thermal trip whose tripping motion can be transmitted to the contact spring by a transmission pin guided in the base part. The thermal trip in the thermal fuse is a fusible material insert, for example, whose tripping motion is effected by the surface of the initially rigid fusible material insert dropping down when an overheating temperature is exceeded. Thus, the fusible material melts, so that the transmission pin can move downwardly under the pressure of the contact spring. Therefore, the tripping motion of the fusible material insert is transmitted to the contact spring.
Such transmission pins are also used in electrical switchgear with other thermal trips, for example in switches that are switched by bimetallic snap discs, capillary tube pressure cells, or other actuators wherein a transmission pin provided in a base of any design transmits a motion of a trip device to a switching member.
To provide reproducible switching conditions in such switchgear, there is a need to match the length of the longitudinally movable transmission pin as exactly as possible to the features of the switchgear, especially to the tolerances of the base part made of insulating material, for example, ceramic, and to the features and design of the tripping elements, for example snap discs and the like. Additional tolerances also appear in the assembly of such a switch, that have to be taken into consideration by a selective and individual cutting to length of the transmission pin. Otherwise the contact conditions may not correspond to the specifications. Contact pressures may be too high or too low or, for example, the opening path of a switch is inadequate.
In the state of the art, the procedure is usually to have available a number of transmission pins of different lengths, to measure the features inside the preassembled switch with a gauge during the assembly process, and then to introduce a pin of suitable length. This necessarily requires the availability of a number of sorted pins of different lengths which then have to be selected during the assembly process and introduced into the preassembled switch. It is complicated and expensive to maintain supplies and to select such pins of different lengths.
Besides this described method, to provide definite switching conditions, it has also been attempted to crush housing parts such as mounting plates and the like in the sense of a positive or negative "length change". Of course, such effects on the housing can be inspected only with difficulty and are poorly reproducible.
It has also already been attempted to vary the distance between the ends of the moving contact and the fixed contact. By crushing the fixed contact more or less against the movable contact kept away from it forcibly, some tolerance compensation can also be achieved, but the selective bending of contact springs has a negative effect on resiliency characteristics.
It is also known to grind pins at their ends during the machining process to achieve exact matching of the pin length to the features of the base or of the other switch components.
Finally, there are also glass pins of changeable length in the state of the art, whose length can be adapted to the housing features by heating and axially oriented crushing pressure.
All of these procedures have drawbacks inasmuch as they require large inventories and especially do not permit fast cycle times during the assembly process.